7. Dropbox

Dropbox reached its $10 billion valuation in 2014 after a $617 million funding round. Since then, the company has shifted its focus to business customers, launching Dropbox Enterprise last year.

The company began to cut back on lavish spending an employee perks in 2016, something that was costing the company $25,000 per year per employee. While Dropbox doesn't disclose its financials, it's reported to have generated more than $500 million in revenue last year. CEO Drew Houston said in June that the company is free cash flow positive now, which is a milestone for a fast-growth tech company.

Various reports have pegged a Dropbox IPO as taking place sometime in 2017.

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6. Pinterest

In May 2015, Pinterest raised a $533 million Series G funding round from investors like Andreessen Horowitz, First Mark, and Goldman Sachs, bumping the San Francisco-based company's valuation up to $11 billion.

The virtual discovery and pinning platform has more than 175 million monthly active users, over half of whom are international. The company has expanded its focus in recent months to overseas markets like the UK, France, Germany, Japan, and Brazil, and has doubled down on increasing the advertising on its platform. Last August, Pinterest acquired online bookmarking service Instapaper.

Rumors swirled throughout 2016 that Pinterest was on the brink of going public, and in October, the company named its first chief financial officer, a signal that Pinterest could be inching closer to an IPO.

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5. SpaceX

After raising a $1 billion funding round from Fidelity and Google in January 2015, Elon Musk's SpaceX reached a $12 billion valuation.

Less than a year later, SpaceX made history when it launched its Falcon 9 rocket into space, dropped off a satellite, then landed back on the ground safely. The success of the launch and landing proved it was possible to reuse expensive rockets, rather than letting them fall into the ocean.

In March of 2017, Airbnb raised a $1 billion funding round that valued the company at $31 billion. Now, Airbnb is reportedly keeping a close eye on Spotify's rumoured route to going public through a direct listing — if it goes well, we may see a publicly traded Airbnb stock next year.

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1. Uber

REUTERS/Danish Siddiqui

Valuation: $68 billion

CEO Travis Kalanick

Uber is the most valuable startup in the US and across the globe, outpacing the valuation of its arch-rival, Didi Chuxing, by nearly $20 billion.

The ride-hailing startup raised several billion dollars in recent years, including $3.5 billion from a Saudi Arabian investment fund and $2 billion in the form of a leveraged loan. Uber announced in May that it lost $708 million in the first quarter of 2017, a narrower loss than the $991 million that Uber lost in the prior three months. The company is also hunting for a new CFO with public company experience, a sign that Uber could be considering going public.

Finances aside, Uber has had a fraught 2017 amid reports of gender discrimination and sexual harassment, a string of executive departures and an acrimonious lawsuit by Waymo, the self-driving car firm spun out of Google, which has accused Uber of stealing its technology.